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Nanocrystalline zinc ferrite photocatalysts formed using the colloid mill and hydrothermal technique
Chemical Engineering Journal 155 (2009) 534–541
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Nanocrystalline zinc ferrite photocatalysts formed using the colloid mill and hydrothermal technique Guoli Fan, Zhijun Gu, Lan Yang, Feng Li ∗ State Key Laboratory of Chemical Resource Engineering, P.O. Box 98, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 27 April 2009 Received in revised form 9 August 2009 Accepted 13 August 2009 Keywords: Zinc ferrite Nanostructure Catalysis Photo-degradation Colloid mill Synthesis
a b s t r a c t Nanocrystalline zinc ferrite (ZnFe2 O4 ) photocatalysts with different crystallite sizes prepared using the colloid mill and hydrothermal technique are reported. This synthetic approach involves a very rapid mixing of Fe3+ cations with reducing agent and reduction process in a colloid mill reactor, followed by a slow oxidation of iron nuclei and structural transformation in a separate hydrothermal process. Material characterization has been presented by powder X-ray diffraction (XRD), chemical element analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and UV–vis diffuse reflectance spectra. The results indicate that ZnFe2 O4 nanocrystals with the uniform crystallite sizes have been obtained using in situ forming iron nuclei as the source of Fe. A possible formation mechanism of ZnFe2 O4 nanocrystals was proposed. Furthermore, through photocatalytic investigation, these ZnFe2 O4 nanocrystals displayed better abilities to photodecompose acid orange II azodye molecule under UV irradiation due to quantum confinement effect and high surface area structure, as compared to bulk ZnFe2 O4 sample prepared by the conventional solid-state method. Since as-synthesized ZnFe2 O4 nanocrystals have excellent chemical and thermal stabilities and exhibit good photocatalytic activities, it can be expected that they may have potential application in the field of industrial photo-degradation of organic azodye pollutants. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, interest in transition metal ferrites of the type MFe2 O4 nanocrystals has greatly increased due to their extensive use in high-density data storage, ferrofluid technology, magnetocaloric refrigeration, magnetic resonance imaging and heterogeneous catalysis [1–3]. For example, zinc ferrite (ZnFe2 O4 ), known as semiconductor material, has a normal spinel structure with tetrahedral sites occupied by Zn2+ ions and octahedral sites by Fe3+ ions [4]. Its chemical and thermal stability has made ZnFe2 O4 important materials, such as magnetic materials [5], catalysts [6–9], and absorbent materials [10,11], in the past decade. Particularly, increasing interests for nanoscale ZnFe2 O4 have been extensively studied by worldwide researchers because of their unique sizedependent physical and chemical properties as compared to bulk counterpart materials [5]. It is well known that nanosized particles are of particular interest for catalytic applications due to their high surface to volume ratio. Several preparation techniques for ZnFe2 O4 nanoparticles with uniform size have been developed including coprecipitation
∗ Corresponding author. Tel.: +86 10 64451226; fax: +86 10 64425385. E-mail address: lifeng [email protected] (F. Li). 1385-8947/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.08.008
[11], sonochemical emulsification and evaporation [12], sol–gel technique [13], hydrothermal synthesis [14,15], mechanical milling [16,17], combustion method [18], reverse micelle technique [19], and organic precursor decomposition techniques [20–22]. However, in most of the above cases, they require handling of large amounts of organic salt, solvent or surfactant, which usually make expensive costs as well as environment pollution, and therefore are not suitable for large-scale industrial applications. On the other hand, it is generally accepted that the formation of homogeneous nanoparticles is strongly dependent on the local distribution of reactants or precursors, which is important for the nucleation and growth of nanoparticles. Therefore, it has always been a challenge or a hot issue to develop affordable and convenient synthesis route for nanoparticles with uniform crystallite size. Recently, we developed a facile pathway for the preparation of magnetic CoFe2 O4 nanocrystals by a simple reduction–oxidation route [23]. This approach offers unique advantages for the uniform synthesis of special complex metal oxide nanoparticles due to the easily achievable high-speed nucleation of metallic precursors in the colloid mill reactor. Semiconductor photocatalysts have been applied widely to degrade the organic pollutants for the remediation of hazardous wastes and contaminated groundwater, and the control of toxic air contaminants [24]. Much attention has been paid to the
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photocatalytic degradation of dyes with semiconductor compounds especially like TiO2 under UV-light. In this contribution, we carefully investigate the uniform synthesis, microstructure and UVlight induced photo-degradation performance of nanocrystalline ZnFe2 O4 with a narrow range of crystallite size. ZnFe2 O4 nanocrystals were prepared via a modified reduction–oxidation route on the basis of the idea that slow oxidation of iron nuclei formed during the reduction in the colloid mill reactor has a remarkable inhibition effect on the growth of ZnFe2 O4 under the hydrothermal conditions. To the best of our knowledge, there is no report on in situ forming iron nuclei precursor as the source of Fe for ZnFe2 O4 nanocrystals. This applied approach, which needs neither the decomposition of complicated organic–metal complex precursors nor a large amount of specialized surfactants as template, is of significant importance in industrial applications as a consequence of low costs, benignancy to environment and synthetic convenience. Furthermore, the photocatalytic results show that resultant nanostructured ZnFe2 O4 displayed better abilities to photodecompose acid orange II azodye molecule under UV irradiation due to quantum confinement effect and high surface area structure, as compared to bulk ZnFe2 O4 sample prepared by the conventional solid-state method. It can be expected that as-synthesized zinc ferrite nanocrystals may have potential application in the field of industrial photo-degradation of organic azodye pollutants due to their excellent structural properties, such as thermal stability and strong resistance to acid and alkali. 2. Experimental 2.1. Synthesis of ZnFe2 O4 nanocrystals In a typical synthesis, three separate solutions were prepared prior to the synthesis. Solution A: analytical-grade Fe(NO3 )3 ·9H2 O were dissolved in 50 mL of deionized water. Different concentrations of Fe3+ ions (0.088, 0.26 and 0.44 M) were used. Solution B: sodium borohydride was dissolved in 25 mL of deionized water to form a solution with the [NaBH4 ]/[Fe3+ ] molar ratio of 2.0. Solution C: analytical-grade Zn(NO3 )2 ·6H2 O were dissolved in 25 mL of deionized water to give a solution ([Zn2+ ] = 1/2[Fe3+ ]). Solutions A and B were simultaneously added rapidly to a colloid mill with rotor speed set at around 6000 rpm and mixed for 2 min, and then the resulting slurry was mixed with solution C. The mixture slurry was sealed in a Teflon-lined autoclave and heated at 120 ◦ C for 12 h, respectively. The suspension was washed with deionized water several times and then ethanol, separately, and the obtained solid was dried at 80 ◦ C for 12 h. 2.2. Characterization Powder X-ray diffraction (XRD) patterns of the samples were collected using a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, graphite-filtered Cu K␣ radiation ( = 0.15418 nm). The samples, as unoriented powders, were step-scanned in steps of 0.04◦ (2) using a count time of 10 s/step. Elemental analysis for metal ions in samples was performed using a Shimadzu ICPS-75000 inductively coupled plasma emission spectrometer (ICP-ES). Samples were dried at 100 ◦ C for 24 h prior to analysis, and solutions were prepared by dissolving the samples in dilute hydrochloric acid (1:1) for 24 h at room temperature. Room temperature Fourier transform infrared (FT-IR) spectra were recorded in the range 4000–400 cm−1 with 2 cm−1 resolution on a Bruker Vector-22 Fourier transform spectrometer using the KBr pellet technique (1 mg of sample in 100 mg of KBr). X-ray photoelectron spectra (XPS) was recorded on a Thermo VG ESCALAB250 X-ray photoelectron spectrometer at a pressure of
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Fig. 1. XRD patterns for ZnFe2 O4 synthesized by using different initial Fe3+ concentrations (a) 0.088 M, (b) 0.26 M and (c) 0.44 M.
about 2 × 10−9 Pa using Al K␣ X-ray as the excitation source. The binding energy (BE) calibration of the spectra has been referred to carbon 1s peak, located at BE = 284.8 eV. Scanning electron microscopy (SEM) microanalyses of the samples were made using a Hitachi S4700 apparatus with the applied voltage of 20 kV, combined with energy dispersive X-ray spectroscopy (EDS) for the determination of metal composition. Transmission electron microscopy (TEM) images were taken using a Hitachi H-800 transmission electron microscopy operated at 100 kV. For TEM analysis, a droplet of the ultrasonically dispersed samples in ethanol was placed onto an amorphous carbon-coated copper grid and then dried at air. High resolution transmission electron microscopy (HRTEM) was carried on a JEM-3010 high resolution transmission electron microscopy at an accelerating voltage of 200 kV. The specific surface area determination was performed by BET method using a Quantachrome Autosorb-1C-VP Analyzer. Solid state UV–vis diffuse reflectance spectra (DRS) were recorded at room temperature and in air by means of a Shimadzu UV-2501PC spectrometer equipped with an integrating sphere attachment using BaSO4 as background. 2.3. Photo-degradation experiments The photo-degradation experiments for acid orange II (AO-II) dye molecule over ZnFe2 O4 ferrites were performed in a quartz reactor at ambient temperature. The UV source was two 36 W of H-type lamps (Beijing electric light sources research institute) with a maximum emission at approximately 254 nm. Typically, an aqueous AO-II solution (20 mg/L, 100 mL) containing 0.05 g of catalyst was vigorously stirred for 0.5 h in the dark to reach absorption–desorption equilibrium before the irradiation. Then, the mixture solution was irradiated by UV-light for a period of time. The extent of dye decomposition was determined by measuring the absorbance value at approximately 484 nm using a Shimadzu UV-2501PC spectrometer. 3. Results and discussion 3.1. Synthesis of ZnFe2 O4 ferrite nanocrystals Fig. 1 displays the powder XRD patterns for three ZnFe2 O4 samples obtained by using different initial concentrations of Fe3+ ions. The high background of the profiles arises due to fluorescence from the Fe atoms [25]. All diffraction peaks can be indexed in a simple cubic lattice and the position along with relative intensity of
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Fig. 2. EDS of ZnFe2 O4 sample synthesized by using initial Fe3+ concentration of 0.088 M.
peaks match well with standard ZnFe2 O4 powder diffraction data (JCPDS 89-4926), which indicates the obtained products have a Fd3m cubic spinel structure and are of high purity. According to the cubic crystal structure of spinel phase, the values of lattice parameter a determined from the most intense (3 1 1) reflection of XRD 2 = (h2 + k2 + l2 )/a2 , h k l = 3 1 1) patterns using the formula (1/dhkl are 0.8441, 0.8439 and 0.8436 nm, respectively, with the increase in concentrations of initial Fe3+ ions, well consistent with that of bulk ZnFe2 O4 (0.8443 nm). Elemental analysis by ICP-ES gives Zn/Fe atomic ratio of 0.48–0.51 in three samples, indicating that these samples are indeed zinc ferrite with the chemical formula of ZnFe2 O4 . In addition, EDS analysis (Fig. 2) shows that the sample using initial Fe3+ concentration of 0.088 M is composed of Zn, Fe and O elements, and the atomic ratio of Zn/Fe/O obtained is very close to 1/2/4 with the expected stoichiometric proportion for ZnFe2 O4 . Also, the presence of C signal within the spectrum arises from external contaminator. The aforementioned results confirm the formation of pure ZnFe2 O4 phase.
On the other hand, the average crystallite size (coherent diffraction domain size) of ZnFe2 O4 samples may be estimated from the values of full-width at half-maximum (FWHM) of the (3 1 1), (4 0 0) and (4 4 0) diffraction peaks by means of the Scherrer equation [L = 0.89/ˇ()cos ] [26], where L is the crystallite size, is the wavelength of the radiation used (0.15418 nm), is the Bragg diffraction angle, and ˇ() is the full-width at halfmaximum (FWHM) of the (3 1 1), (4 0 0), and (4 4 0) reflections. FWHM is modified for instrumental broadening by the formula B = B(M) − B2 (S)/B(M) [27], where B(M) is FWHM of a given reflection for the prepared sample and B(S) is that of a reflection taken for a standard SiC sample with a known average crystallite size larger than 10 m. With the increase of the initial metal concentration, the estimated average crystallite size for the ZnFe2 O4 samples is about 9.0, 14.0 and 19.0 nm, respectively. The morphological characteristics of the obtained ZnFe2 O4 were investigated by SEM and TEM analysis. SEM photographs of ZnFe2 O4 samples (Fig. 3) reveal that all samples exhibit a compact arrangement of homogeneous nanoparticles with roughly spherical shape. TEM micrographs (Fig. 4) also confirm the nature of uniform particle size. Indeed, each sample is formed by particles, the average crystallite sizes (from the counting of about 100 particles) of ZnFe2 O4 nanoparticles are 9 ± 3, 15 ± 6 and 22 ± 5 nm, respectively. Since the average crystallite size estimated from TEM micrographs and the coherent diffraction domain size inferred from XRD results are in good agreement, one could say that ZnFe2 O4 nanoparticles are monodisperse nanocrystals. The HRTEM and selected area electron diffraction (SAED) analysis (Fig. 5) also provides more detailed structural information about the ZnFe2 O4 ferrite. A typical HRTEM image of an individual ZnFe2 O4 nanocrystal indicates interplanar distance of 0.299 nm that are characteristic of (2 2 0) spinel planes. Moreover, the SAED pattern taken from several individual 9 nm ZnFe2 O4 nanocrystals reveals that the positions of discernible diffraction rings match well with standard ZnFe2 O4 powder diffraction data. The above results support the nanocrystalline and monodisperse nature of as-synthesized ZnFe2 O4 samples. Unlike ZnO/Fe2 O3 precursors used as the source of Zn and Fe employed for the synthesis of ZnFe2 O4 in the solid-state reactions, Zn2+ /Fe precursors were alternatively employed in the present sys-
Fig. 3. SEM images for ZnFe2 O4 samples with different crystallite sizes of (a) 9 nm, (b) 14 nm and (c) 19 nm.
Fig. 4. TEM images for ZnFe2 O4 samples with different crystallite sizes of (a) 9 nm, (b) 14 nm and (c) 19 nm.
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Fig. 6. XRD patterns for reduced immediate products in the colloid mill (a) without aging and (b) aging for 3 h at room temperature under N2 atmosphere. Fig. 5. HRTEM image of 9 nm ZnFe2 O4 sample. The inset pattern is the electron diffraction pattern.
tem. Herein, the mixing of Fe3+ cations with reducing agent and nucleation of metallic iron are completed within a very short time in the vigorously stirring colloid mill reactor, and thus the process that nucleation of iron and nuclei growth take place simultaneously can be avoided, which facilitates to obtain nuclei with a narrow range of crystallite sizes [28]. In order to understand the mechanism of synthesis, XRD analysis for the reduced intermediate product collected from the colloid mill was carried out. XRD pattern only reveals the presence of amorphous phase (Fig. 6a). Moreover, after the reduced intermediate product is further aged for 3 h at room temperature under N2 atmosphere, XRD pattern of resulting sample exhibits weak characteristic (1 1 0) and (2 0 0) reflections of body-centered cubic (bcc) ␣-Fe (Fig. 6b), consistent with the reported data (JCPDS 06-0696). The aforementioned results further indicate that iron nuclei can form through rapid reduction of NaBH4 in the colloid mill. On the other hand, the reduction takes place in a thin liquid film under conditions of high-speed fluid shear and high pressures and friction between the stator and rotor (rotating at 6000 rpm) with the mixture also being subjected to intensive vibration. In this highly turbulent zone, the newly formed metallic iron nuclei undergo energetic collisions and this, together with the strong hydraulic shear forces to which the nucleation mixture is subjected, means that agglomeration of the nuclei of iron is inhibited and their sizes remain at a minimum. As a result, the mechanism of ZnFe2 O4
ferrite formation is proposed as follows (Eqs. (1)–(3)): 4Fe3+ + 3BH4 − + 12H2 O → 4Fe + 3B(OH)4 − + 6H2 + 12H+
(1)
4Fe + 3O2 + 6H2 O → 4Fe(OH)3
(2)
2Fe(OH)3 + Zn
2+
→ ZnFe2 O4 + 2H2 O + 2H
+
(3)
The formation mechanism for ZnFe2 O4 nanocrystals is proposed and illustrated schematically in Fig. 7. In the initial step, the Fe3+ cations were rapidly reduced to metallic iron nuclei in the colloid mill (Eq. (1)), using NaBH4 as reducing agent [29]. Then, the formed metallic iron nuclei reacted through a slow oxidation with the residual trace amount of dissolved oxygen and transform into corresponding Fe(OH)3 compounds under the hydrothermal conditions (Eq. (2)). At last, the newly formed Fe(OH)3 diffused into the Zn2+ -containing solution and reacted with Zn2+ ions, through a dehydration reaction under hydrothermal conditions [30], thus giving rise to ZnFe2 O4 nanocrystals with the uniform crystallite sizes (Eq. (3)). Here, slow oxidation of iron nuclei exhibits a remarkable inhibition effect on the growth of ZnFe2 O4 . Additionally, in this method, the larger concentration of Fe3+ cations could favor the growth of larger ferrite nanocrystals. It is because the increase in the metal concentration could lead to the formation of a larger amount of metallic iron nuclei in the colloid mill reactor. Consequently, during the hydrothermal process, due to the limited trace amount of dissolved oxygen, the nucleation rate of ferrite nuclei, which are formed through the slow oxidation of metal nuclei followed by the diffusion of Fe(OH)3 , is almost same for each sample. At last, the
Fig. 7. Schematic illustration of the synthesis of ZnFe2 O4 nanocrystals.
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dissolution of a larger amount of iron nuclei is advantageous to the growth of ferrite crystallites, thus leading to the formation of ZnFe2 O4 ferrite with larger crystallite size. 3.2. Structural properties of ZnFe2 O4 nanocrystals
Fig. 8. FT-IR spectra for ZnFe2 O4 samples with different crystallite sizes of (a) 9 nm, (b) 14 nm, (c) 19 nm.
The presence of the spinel structure for ZnFe2 O4 samples is further confirmed by FT-IR spectroscopy (Fig. 8). There are two strong characteristic absorption peaks at about 550 and 415 cm−1 , which can be assigned to the stretching vibrations of the Zn–O bonds in tetrahedral positions and the Fe–O bonds in octahedral positions, respectively [31]. In addition, it is noteworthy that with the increase of crystallite size, a slight shift for the stretching vibration of the Zn–O bonds from 553 to 542 cm−1 can be observed. It is well documented that the decrease in crystallite size of ZnFe2 O4 nanoparticles favors the redistribution of Zn2+ cations to octahedral sites in spinel structure [32], in spite of the high affinity of Zn2+ ions in tetrahedral sites. In addition, it is well known that the tetrahedrally coordinated Zn–O distance is shorter than octahedrally coordinated Zn–O distance in cubic spinel structure. Therefore, with the decreasing crystallite size of ZnFe2 O4 nanoparticles, the number of octahedrally coordinated Zn–O bonds increase gradually, thus leading to the observed shift of Zn–O vibration in IR spectra. On the other hand, in view of crystalline electric field effect and size effect, the crystals with smaller crystallite size have more intensive distortion of lattices and higher strength of surface chemical bonds [33], also leading to the change in IR spectra to some extent.
Fig. 9. XPS spectra of 9 nm ZnFe2 O4 sample: (a) survey (b) Zn 2p and (c) Fe 2p.
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where Eg represents the optical band gap, Ep is the photon energy, K is a constant, and n depends on the nature of the transition. In fact, n assumes a value of 1/2, 3/2, 2, and 3 for allowed direct, forbidden direct, allowed indirect, forbidden indirect transitions, respectively. By analysis of absorption coefficients for ZnFe2 O4 , the optical energy gaps can be estimated using a classical Tauc approach [38]. As for three ZnFe2 O4 nanocrystal samples, the best fit of (˛Ep )2 vs Ep was obtained for n = 1/2 (Fig. 11), suggesting the allowed direct transitions across the energy band gap of ZnFe2 O4 . The extrapolated value (the straight lines to the x axis) of Ep at ˛ = 0 gives absorption edge energies corresponding to energy band gap (Eg ) of 2.10 for 9 nm ZnFe2 O4 sample, 2.00 for 14 nm sample and 1.92 eV for 19 nm sample, respectively. The noteworthy feature is a direct relationship between the grain size of the nanoparticles and optical band gap. The shift in Eg of the ZnFe2 O4 nanoparticles with the increasing crystallite sizes is the result of quantum confinement effects arising from the small size regime [38,39]. 3.3. Photo-degradation properties of ZnFe2 O4 nanocrystals Fig. 10. UV–vis diffuse reflectance spectra of ZnFe2 O4 with different crystallite sizes of (a) 9 nm, (b) 14 nm and (c) 19 nm.
The surface/near-surface chemical states of the obtained 9 nm ZnFe2 O4 sample are analyzed by XPS within a range of binding energies of 0–1200 eV (Fig. 9a). Core levels of Zn 2p and Fe 2p can be identified, and no contaminant species are detectable within the sensitivity of the technique. Only a very low peak from absorbed carbon is present on the spectra. The fine spectra of the Zn 2p and Fe 2p peaks are displayed in Figs. 9b and c, respectively. The binding energy of the Zn 2p3/2 peak is 1021.1 eV, indicative of the presence of Zn2+ species. The Fe 2p3/2 and Fe 2p1/2 spectrum obtained from the present study generally shows two distinguishable main peaks at BE of around 711.0 and 724.9 eV, respectively, accompanied by two corresponding satellite peaks for Fe 2p3/2 and Fe 2p1/2 visible at BE of around 718.9 and 733.0 eV, only indicative of the presence of Fe3+ cations [34,35]. Generally, the chemical states of the species characterized by means of an ordinary XPS route only should be considered representative of the surface or external layers composition of the synthesized ferrite samples. However, by integrating the core-level intensities, the obtained Zn/Fe atomic ratio is 0.52, very close to that obtained by ICP and EDS analysis. This further reflects the homogeneous nature of as-synthesized ZnFe2 O4 nanocrystals. To give important information concerning the effect of crystallite size on the optical properties of ZnFe2 O4 semiconductor materials, the UV–vis diffuse reflectance measurement was carried out (Fig. 10). The optical absorbance can be approximately calculated from the optical reflectance data by the Kubelka–Munk function, ˛ = (1 − R)2 /2R, where ˛ is the absorption coefficient, R is the diffuse reflectance [36]. As shown in Fig. 10, the as-synthesized ZnFe2 O4 nanocrystals are highly photoresponsive in both the UV and visible light ranges (>400 nm) as expected. Especially, in the wavelength range of 300–500 nm, three samples present the strong light absorbance (in term of Kubelka–Munk equivalent absorbance units), indicative of wider absorption band than TiO2 . It is well known that when semiconductors are irradiated by photons with energy similar to the band gap, they can yield an electron (e−) in the conduction band and a hole (h+) in the valence band. For a large number of semiconductors, the dependence of the absorption coefficient ˛, for the high-frequency region, upon the photon energy Ep , for optically induced transitions, is usually given by the following expression [37]: ˛Ep = K(Ep − Eg )n
(4)
As discussed above, as-synthesized ZnFe2 O4 nanocrystal is a semiconductor with the optical adsorption threshold in the range of 590–645 nm. Photocatalytic process is based on the generation of electron/hole pairs by means of band-gap radiation, which can give rise to redox reactions with species adsorbed on the surface of the catalysts. In addition, the BET analysis shows that the specific surface areas of ZnFe2 O4 nanocrystals are 54.2, 63.5, 100.5 m2 /g, respectively, with the decrease of crystallite size. Therefore it is likely that due to their suitable band gap from the UV-light response discussed above and high specific surface area, the as-synthesized ZnFe2 O4 nanocrystals produce large amounts of electron/hole pairs under UV irradiation with proper wavelength, and thus may show good photocatalytic performance. ZnFe2 O4 sample with 5 m2 g−1 of specific surface area prepared by the conventional solid-state method at 1000 ◦ C for 12 h is used here as a reference sample, making direct comparison of the catalytic activity of the two kinds of zinc ferrite materials possible. Temporal evolution of the spectral changes taking place during the photo-degradation of AO-II azodye molecule mediated by 9 nm ZnFe2 O4 nanocrystal is displayed in Fig. 12. As expected, a gradual decrease in the intensity of strong absorption band at 484 nm for AO-II solutions due to the n–* transitions of azo bond
Fig. 11. Tauc plot for ZnFe2 O4 with different crystallite sizes of (a) 9 nm, (b) 14 nm and (c) 19 nm.
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Fig. 12. Absorption changes of AO-II solution during the photo-degradation process over 9 nm ZnFe2 O4 sample under UV-light irradiation.
is observed during the course of the photoassisted degradation, indicating that the azo groups has been destroyed [40]. Meanwhile, the other absorption at 309 and 230 nm attributed to the –* transition related to aromatic rings decreases significantly along with irradiation time and no new bands appear, implying that full oxidative decomposition had occurred and that other intermediates are no longer formed. The above result indicates that the ZnFe2 O4 nanocrystals indeed exhibit good activity for the photodegradation of AO-II under UV-light irradiation. The dependence of AO-II photo-degradation on the crystallite size of ZnFe2 O4 nanocrystals under UV-light irradiation has been investigated at the same operating conditions. As shown in Fig. 13, three ZnFe2 O4 nanocrystals with different crystallite sizes exhibit good photo-degradation activity for AO-II, and there are no obvious degradation of dye molecules was observed in the darkness. It is found that the photocatalytic activity is gradually enhanced with decreasing crystallite size. Especially, the degradation percentage of AO-II after 6 h on 9 nm ZnFe2 O4 sample was as high as 97%. The dependence of AO-II photo-degradation on the crystallite size of ZnFe2 O4 nanocrystals can be figured out more clearly from Fig. 14, illustrating that the degradation percentage of
Fig. 14. Influence of the crystallite size of ZnFe2 O4 sample on the degradation percentage of AO-II and kinetic rate constant after 6 h under UV-light irradiation.
AO-II and kinetic rate constant k (first-order) basing on the Langmuir–Hinshelwood model increase obviously with the crystallite size of ZnFe2 O4 nanocrystals. Firstly, it is mainly because the generated conduction band of nanosized ZnFe2 O4 semiconductor with the larger optical band gap is more negative and the valence band is more positive, which makes the oxidation and reduction abilities of photogenerated holes increasing [8,41]. Secondly, the smaller size can lead to a shorter time for the electron diffusion from bulk phase to interface, hence decreases the probability of holes coupling with electrons [42,43]. Finally, the higher surface areas also facilitates to the absorption of dye molecules on the surfaces of ZnFe2 O4 , which makes the free radical more chance to touch with dye molecules thus giving a faster reaction speed. On the other hand, it should be noted that all ZnFe2 O4 samples prepared by modified reduction–oxidation route show higher conversion of AOII dye during the course of photo-degradation, as compared to bulk ZnFe2 O4 sample prepared by the conventional solid-state method, due to the large particle size and low surface area of bulk ZnFe2 O4 sample. In addition, as discussed above, as-synthesized ZnFe2 O4 nanocrystals can be excited under visible light, which expands their utilization scope. Further work on the visible-light derived photocatalytic properties of nanocrystalline ZnFe2 O4 materials is currently underway in our laboratory. 4. Conclusions
Fig. 13. Photo-degradation of AO-II monitored as the normalized concentration change vs irradiation time using series samples under UV-light irradiation: (a) 9 nm ZnFe2 O4 , (b) 14 nm ZnFe2 O4 , (c) 19 nm ZnFe2 O4 , (d) bulk ZnFe2 O4 and (e) dark reaction with 9 nm ZnFe2 O4 .
Zinc ferrite nanocrystals with the average crystallite sizes of 9, 14 and 19 nm were successfully prepared by a modified reduction–oxidation method, which involves a very rapid mixing of ferric cations with reducing agent and nucleation of metallic iron by reduction within a very short time in a colloid mill reactor, followed by slow oxidation of iron and structural transformation under hydrothermal conditions. It is believed that the extreme forces, to which the nucleation of metallic iron obtained with a very short time was subjected in the colloid mill, prevented aggregation of the newly formed iron nuclei. Consequently, when the resulting iron nuclei were slowly oxidized in a Zn2+ -containing aqueous solution under separate hydrothermal treatment, wellcrystallized ZnFe2 O4 nanocrystals with narrow size distribution were obtained. Meanwhile, the photo-degradation investigation reveals that as-synthesized ZnFe2 O4 nanocrystals displayed good abilities to photodecompose acid orange II under UV irradiation,
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and that the reduced crystallite sizes of ZnFe2 O4 nanocrystals can lead to an enhanced photocatalytic activity, proceeding from the quantum confinement effect and high specific surface area structure. The findings in this work are believed to provide further insights to understanding of relation between the structure of ZnFe2 O4 nanocrystals and their photocatalytic properties, which is significantly crucial to generate the desired photocatalyst. It can be expected that as-synthesized zinc ferrite nanocrystals with excellent structural properties, such as high thermal stability and strong resistance to acid and alkali, may have potential applications in the field of industrial photo-degradation of organic azodye pollutants. Acknowledgements We gratefully thank the financial support from the National Natural Science Foundation of China, 973 Program (2009CB939802), 111 Project (No. B07004) and Changjiang Scholars and Innovative Research Team in Universities (IRT 0406). References [1] P. Boldrin, A.K. Hebb, A.A. Chaudhry, L. Otley, B. Thiebaut, P. Bishop, J. Darr, Ind. Eng. Chem. Res. 46 (2007) 4830–4838. [2] H.H. Hamdeh, Z. Xia, R. Foehrweiser, B.J. McCormick, R.J. Willey, G. Busca, J. Appl. Phys. 76 (1994) 1135–1140. [3] W.Z. Xiong, G.M. Kale, Anal. Chem. 79 (2007) 3561–3567. [4] H. Deng, X.L. Li, Q. Peng, X. Wang, J.P. Chen, Y.D. Li, Angew. Chem. Int. Ed. 44 (2005) 2782–2785. [5] C.W. Yao, Q.S. Zeng, G.F. Goya, T. Torres, J.F. Liu, H.P. Wu, M.Y. Ge, Y.W. Zeng, Y.W. Wang, J.Z. Jiang, J. Phys. Chem. C 111 (2007) 12274–12278. [6] J.J. Liu, G.X. Lu, H.L. He, H. Tan, T. Xu, K. Xu, Mater. Res. Bull. 31 (1996) 1049–1056. [7] J.A. Toledo-Antonio, N. Nava, M. Martínez, X. Bokhimi, Appl. Catal. A 234 (2002) 137–144. [8] J. Qiu, C. Wang, M. Gu, Mater. Sci. Eng. B 112 (2004) 1–4. [9] Z.H. Yuan, L.D. Zhang, J. Mater. Chem. 11 (2001) 1265–1268. [10] M. Kobayashi, H. Shirai, M. Nunokawa, Ind. Eng. Chem. Res. 41 (2002) 2903–2909. [11] M. Tsukada, K. Abe, Y. Yonemochi, A. Ameyama, H. Kamiya, S. Kambara, H. Moritomi, T. Uehara, Powder Technol. 180 (2008) 232–238. [12] M. Sivakumar, T. Takami, H. Ikuta, A. Towata, K. Yasui, T. Tuziuti, T. Kozuka, D. Bhattacharya, Y. Iida, J. Phys. Chem. B 110 (2006) 15234–15243.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Nanocrystalline zinc ferrite photocatalysts formed using the colloid mill and hydrothermal technique Guoli Fan, Zhijun Gu, Lan Yang, Feng Li ∗ State Key Laboratory of Chemical Resource Engineering, P.O. Box 98, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 27 April 2009 Received in revised form 9 August 2009 Accepted 13 August 2009 Keywords: Zinc ferrite Nanostructure Catalysis Photo-degradation Colloid mill Synthesis
a b s t r a c t Nanocrystalline zinc ferrite (ZnFe2 O4 ) photocatalysts with different crystallite sizes prepared using the colloid mill and hydrothermal technique are reported. This synthetic approach involves a very rapid mixing of Fe3+ cations with reducing agent and reduction process in a colloid mill reactor, followed by a slow oxidation of iron nuclei and structural transformation in a separate hydrothermal process. Material characterization has been presented by powder X-ray diffraction (XRD), chemical element analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and UV–vis diffuse reflectance spectra. The results indicate that ZnFe2 O4 nanocrystals with the uniform crystallite sizes have been obtained using in situ forming iron nuclei as the source of Fe. A possible formation mechanism of ZnFe2 O4 nanocrystals was proposed. Furthermore, through photocatalytic investigation, these ZnFe2 O4 nanocrystals displayed better abilities to photodecompose acid orange II azodye molecule under UV irradiation due to quantum confinement effect and high surface area structure, as compared to bulk ZnFe2 O4 sample prepared by the conventional solid-state method. Since as-synthesized ZnFe2 O4 nanocrystals have excellent chemical and thermal stabilities and exhibit good photocatalytic activities, it can be expected that they may have potential application in the field of industrial photo-degradation of organic azodye pollutants. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, interest in transition metal ferrites of the type MFe2 O4 nanocrystals has greatly increased due to their extensive use in high-density data storage, ferrofluid technology, magnetocaloric refrigeration, magnetic resonance imaging and heterogeneous catalysis [1–3]. For example, zinc ferrite (ZnFe2 O4 ), known as semiconductor material, has a normal spinel structure with tetrahedral sites occupied by Zn2+ ions and octahedral sites by Fe3+ ions [4]. Its chemical and thermal stability has made ZnFe2 O4 important materials, such as magnetic materials [5], catalysts [6–9], and absorbent materials [10,11], in the past decade. Particularly, increasing interests for nanoscale ZnFe2 O4 have been extensively studied by worldwide researchers because of their unique sizedependent physical and chemical properties as compared to bulk counterpart materials [5]. It is well known that nanosized particles are of particular interest for catalytic applications due to their high surface to volume ratio. Several preparation techniques for ZnFe2 O4 nanoparticles with uniform size have been developed including coprecipitation
∗ Corresponding author. Tel.: +86 10 64451226; fax: +86 10 64425385. E-mail address: lifeng [email protected] (F. Li). 1385-8947/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.08.008
[11], sonochemical emulsification and evaporation [12], sol–gel technique [13], hydrothermal synthesis [14,15], mechanical milling [16,17], combustion method [18], reverse micelle technique [19], and organic precursor decomposition techniques [20–22]. However, in most of the above cases, they require handling of large amounts of organic salt, solvent or surfactant, which usually make expensive costs as well as environment pollution, and therefore are not suitable for large-scale industrial applications. On the other hand, it is generally accepted that the formation of homogeneous nanoparticles is strongly dependent on the local distribution of reactants or precursors, which is important for the nucleation and growth of nanoparticles. Therefore, it has always been a challenge or a hot issue to develop affordable and convenient synthesis route for nanoparticles with uniform crystallite size. Recently, we developed a facile pathway for the preparation of magnetic CoFe2 O4 nanocrystals by a simple reduction–oxidation route [23]. This approach offers unique advantages for the uniform synthesis of special complex metal oxide nanoparticles due to the easily achievable high-speed nucleation of metallic precursors in the colloid mill reactor. Semiconductor photocatalysts have been applied widely to degrade the organic pollutants for the remediation of hazardous wastes and contaminated groundwater, and the control of toxic air contaminants [24]. Much attention has been paid to the
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photocatalytic degradation of dyes with semiconductor compounds especially like TiO2 under UV-light. In this contribution, we carefully investigate the uniform synthesis, microstructure and UVlight induced photo-degradation performance of nanocrystalline ZnFe2 O4 with a narrow range of crystallite size. ZnFe2 O4 nanocrystals were prepared via a modified reduction–oxidation route on the basis of the idea that slow oxidation of iron nuclei formed during the reduction in the colloid mill reactor has a remarkable inhibition effect on the growth of ZnFe2 O4 under the hydrothermal conditions. To the best of our knowledge, there is no report on in situ forming iron nuclei precursor as the source of Fe for ZnFe2 O4 nanocrystals. This applied approach, which needs neither the decomposition of complicated organic–metal complex precursors nor a large amount of specialized surfactants as template, is of significant importance in industrial applications as a consequence of low costs, benignancy to environment and synthetic convenience. Furthermore, the photocatalytic results show that resultant nanostructured ZnFe2 O4 displayed better abilities to photodecompose acid orange II azodye molecule under UV irradiation due to quantum confinement effect and high surface area structure, as compared to bulk ZnFe2 O4 sample prepared by the conventional solid-state method. It can be expected that as-synthesized zinc ferrite nanocrystals may have potential application in the field of industrial photo-degradation of organic azodye pollutants due to their excellent structural properties, such as thermal stability and strong resistance to acid and alkali. 2. Experimental 2.1. Synthesis of ZnFe2 O4 nanocrystals In a typical synthesis, three separate solutions were prepared prior to the synthesis. Solution A: analytical-grade Fe(NO3 )3 ·9H2 O were dissolved in 50 mL of deionized water. Different concentrations of Fe3+ ions (0.088, 0.26 and 0.44 M) were used. Solution B: sodium borohydride was dissolved in 25 mL of deionized water to form a solution with the [NaBH4 ]/[Fe3+ ] molar ratio of 2.0. Solution C: analytical-grade Zn(NO3 )2 ·6H2 O were dissolved in 25 mL of deionized water to give a solution ([Zn2+ ] = 1/2[Fe3+ ]). Solutions A and B were simultaneously added rapidly to a colloid mill with rotor speed set at around 6000 rpm and mixed for 2 min, and then the resulting slurry was mixed with solution C. The mixture slurry was sealed in a Teflon-lined autoclave and heated at 120 ◦ C for 12 h, respectively. The suspension was washed with deionized water several times and then ethanol, separately, and the obtained solid was dried at 80 ◦ C for 12 h. 2.2. Characterization Powder X-ray diffraction (XRD) patterns of the samples were collected using a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, graphite-filtered Cu K␣ radiation ( = 0.15418 nm). The samples, as unoriented powders, were step-scanned in steps of 0.04◦ (2) using a count time of 10 s/step. Elemental analysis for metal ions in samples was performed using a Shimadzu ICPS-75000 inductively coupled plasma emission spectrometer (ICP-ES). Samples were dried at 100 ◦ C for 24 h prior to analysis, and solutions were prepared by dissolving the samples in dilute hydrochloric acid (1:1) for 24 h at room temperature. Room temperature Fourier transform infrared (FT-IR) spectra were recorded in the range 4000–400 cm−1 with 2 cm−1 resolution on a Bruker Vector-22 Fourier transform spectrometer using the KBr pellet technique (1 mg of sample in 100 mg of KBr). X-ray photoelectron spectra (XPS) was recorded on a Thermo VG ESCALAB250 X-ray photoelectron spectrometer at a pressure of
535
Fig. 1. XRD patterns for ZnFe2 O4 synthesized by using different initial Fe3+ concentrations (a) 0.088 M, (b) 0.26 M and (c) 0.44 M.
about 2 × 10−9 Pa using Al K␣ X-ray as the excitation source. The binding energy (BE) calibration of the spectra has been referred to carbon 1s peak, located at BE = 284.8 eV. Scanning electron microscopy (SEM) microanalyses of the samples were made using a Hitachi S4700 apparatus with the applied voltage of 20 kV, combined with energy dispersive X-ray spectroscopy (EDS) for the determination of metal composition. Transmission electron microscopy (TEM) images were taken using a Hitachi H-800 transmission electron microscopy operated at 100 kV. For TEM analysis, a droplet of the ultrasonically dispersed samples in ethanol was placed onto an amorphous carbon-coated copper grid and then dried at air. High resolution transmission electron microscopy (HRTEM) was carried on a JEM-3010 high resolution transmission electron microscopy at an accelerating voltage of 200 kV. The specific surface area determination was performed by BET method using a Quantachrome Autosorb-1C-VP Analyzer. Solid state UV–vis diffuse reflectance spectra (DRS) were recorded at room temperature and in air by means of a Shimadzu UV-2501PC spectrometer equipped with an integrating sphere attachment using BaSO4 as background. 2.3. Photo-degradation experiments The photo-degradation experiments for acid orange II (AO-II) dye molecule over ZnFe2 O4 ferrites were performed in a quartz reactor at ambient temperature. The UV source was two 36 W of H-type lamps (Beijing electric light sources research institute) with a maximum emission at approximately 254 nm. Typically, an aqueous AO-II solution (20 mg/L, 100 mL) containing 0.05 g of catalyst was vigorously stirred for 0.5 h in the dark to reach absorption–desorption equilibrium before the irradiation. Then, the mixture solution was irradiated by UV-light for a period of time. The extent of dye decomposition was determined by measuring the absorbance value at approximately 484 nm using a Shimadzu UV-2501PC spectrometer. 3. Results and discussion 3.1. Synthesis of ZnFe2 O4 ferrite nanocrystals Fig. 1 displays the powder XRD patterns for three ZnFe2 O4 samples obtained by using different initial concentrations of Fe3+ ions. The high background of the profiles arises due to fluorescence from the Fe atoms [25]. All diffraction peaks can be indexed in a simple cubic lattice and the position along with relative intensity of
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Fig. 2. EDS of ZnFe2 O4 sample synthesized by using initial Fe3+ concentration of 0.088 M.
peaks match well with standard ZnFe2 O4 powder diffraction data (JCPDS 89-4926), which indicates the obtained products have a Fd3m cubic spinel structure and are of high purity. According to the cubic crystal structure of spinel phase, the values of lattice parameter a determined from the most intense (3 1 1) reflection of XRD 2 = (h2 + k2 + l2 )/a2 , h k l = 3 1 1) patterns using the formula (1/dhkl are 0.8441, 0.8439 and 0.8436 nm, respectively, with the increase in concentrations of initial Fe3+ ions, well consistent with that of bulk ZnFe2 O4 (0.8443 nm). Elemental analysis by ICP-ES gives Zn/Fe atomic ratio of 0.48–0.51 in three samples, indicating that these samples are indeed zinc ferrite with the chemical formula of ZnFe2 O4 . In addition, EDS analysis (Fig. 2) shows that the sample using initial Fe3+ concentration of 0.088 M is composed of Zn, Fe and O elements, and the atomic ratio of Zn/Fe/O obtained is very close to 1/2/4 with the expected stoichiometric proportion for ZnFe2 O4 . Also, the presence of C signal within the spectrum arises from external contaminator. The aforementioned results confirm the formation of pure ZnFe2 O4 phase.
On the other hand, the average crystallite size (coherent diffraction domain size) of ZnFe2 O4 samples may be estimated from the values of full-width at half-maximum (FWHM) of the (3 1 1), (4 0 0) and (4 4 0) diffraction peaks by means of the Scherrer equation [L = 0.89/ˇ()cos ] [26], where L is the crystallite size, is the wavelength of the radiation used (0.15418 nm), is the Bragg diffraction angle, and ˇ() is the full-width at halfmaximum (FWHM) of the (3 1 1), (4 0 0), and (4 4 0) reflections. FWHM is modified for instrumental broadening by the formula B = B(M) − B2 (S)/B(M) [27], where B(M) is FWHM of a given reflection for the prepared sample and B(S) is that of a reflection taken for a standard SiC sample with a known average crystallite size larger than 10 m. With the increase of the initial metal concentration, the estimated average crystallite size for the ZnFe2 O4 samples is about 9.0, 14.0 and 19.0 nm, respectively. The morphological characteristics of the obtained ZnFe2 O4 were investigated by SEM and TEM analysis. SEM photographs of ZnFe2 O4 samples (Fig. 3) reveal that all samples exhibit a compact arrangement of homogeneous nanoparticles with roughly spherical shape. TEM micrographs (Fig. 4) also confirm the nature of uniform particle size. Indeed, each sample is formed by particles, the average crystallite sizes (from the counting of about 100 particles) of ZnFe2 O4 nanoparticles are 9 ± 3, 15 ± 6 and 22 ± 5 nm, respectively. Since the average crystallite size estimated from TEM micrographs and the coherent diffraction domain size inferred from XRD results are in good agreement, one could say that ZnFe2 O4 nanoparticles are monodisperse nanocrystals. The HRTEM and selected area electron diffraction (SAED) analysis (Fig. 5) also provides more detailed structural information about the ZnFe2 O4 ferrite. A typical HRTEM image of an individual ZnFe2 O4 nanocrystal indicates interplanar distance of 0.299 nm that are characteristic of (2 2 0) spinel planes. Moreover, the SAED pattern taken from several individual 9 nm ZnFe2 O4 nanocrystals reveals that the positions of discernible diffraction rings match well with standard ZnFe2 O4 powder diffraction data. The above results support the nanocrystalline and monodisperse nature of as-synthesized ZnFe2 O4 samples. Unlike ZnO/Fe2 O3 precursors used as the source of Zn and Fe employed for the synthesis of ZnFe2 O4 in the solid-state reactions, Zn2+ /Fe precursors were alternatively employed in the present sys-
Fig. 3. SEM images for ZnFe2 O4 samples with different crystallite sizes of (a) 9 nm, (b) 14 nm and (c) 19 nm.
Fig. 4. TEM images for ZnFe2 O4 samples with different crystallite sizes of (a) 9 nm, (b) 14 nm and (c) 19 nm.
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Fig. 6. XRD patterns for reduced immediate products in the colloid mill (a) without aging and (b) aging for 3 h at room temperature under N2 atmosphere. Fig. 5. HRTEM image of 9 nm ZnFe2 O4 sample. The inset pattern is the electron diffraction pattern.
tem. Herein, the mixing of Fe3+ cations with reducing agent and nucleation of metallic iron are completed within a very short time in the vigorously stirring colloid mill reactor, and thus the process that nucleation of iron and nuclei growth take place simultaneously can be avoided, which facilitates to obtain nuclei with a narrow range of crystallite sizes [28]. In order to understand the mechanism of synthesis, XRD analysis for the reduced intermediate product collected from the colloid mill was carried out. XRD pattern only reveals the presence of amorphous phase (Fig. 6a). Moreover, after the reduced intermediate product is further aged for 3 h at room temperature under N2 atmosphere, XRD pattern of resulting sample exhibits weak characteristic (1 1 0) and (2 0 0) reflections of body-centered cubic (bcc) ␣-Fe (Fig. 6b), consistent with the reported data (JCPDS 06-0696). The aforementioned results further indicate that iron nuclei can form through rapid reduction of NaBH4 in the colloid mill. On the other hand, the reduction takes place in a thin liquid film under conditions of high-speed fluid shear and high pressures and friction between the stator and rotor (rotating at 6000 rpm) with the mixture also being subjected to intensive vibration. In this highly turbulent zone, the newly formed metallic iron nuclei undergo energetic collisions and this, together with the strong hydraulic shear forces to which the nucleation mixture is subjected, means that agglomeration of the nuclei of iron is inhibited and their sizes remain at a minimum. As a result, the mechanism of ZnFe2 O4
ferrite formation is proposed as follows (Eqs. (1)–(3)): 4Fe3+ + 3BH4 − + 12H2 O → 4Fe + 3B(OH)4 − + 6H2 + 12H+
(1)
4Fe + 3O2 + 6H2 O → 4Fe(OH)3
(2)
2Fe(OH)3 + Zn
2+
→ ZnFe2 O4 + 2H2 O + 2H
+
(3)
The formation mechanism for ZnFe2 O4 nanocrystals is proposed and illustrated schematically in Fig. 7. In the initial step, the Fe3+ cations were rapidly reduced to metallic iron nuclei in the colloid mill (Eq. (1)), using NaBH4 as reducing agent [29]. Then, the formed metallic iron nuclei reacted through a slow oxidation with the residual trace amount of dissolved oxygen and transform into corresponding Fe(OH)3 compounds under the hydrothermal conditions (Eq. (2)). At last, the newly formed Fe(OH)3 diffused into the Zn2+ -containing solution and reacted with Zn2+ ions, through a dehydration reaction under hydrothermal conditions [30], thus giving rise to ZnFe2 O4 nanocrystals with the uniform crystallite sizes (Eq. (3)). Here, slow oxidation of iron nuclei exhibits a remarkable inhibition effect on the growth of ZnFe2 O4 . Additionally, in this method, the larger concentration of Fe3+ cations could favor the growth of larger ferrite nanocrystals. It is because the increase in the metal concentration could lead to the formation of a larger amount of metallic iron nuclei in the colloid mill reactor. Consequently, during the hydrothermal process, due to the limited trace amount of dissolved oxygen, the nucleation rate of ferrite nuclei, which are formed through the slow oxidation of metal nuclei followed by the diffusion of Fe(OH)3 , is almost same for each sample. At last, the
Fig. 7. Schematic illustration of the synthesis of ZnFe2 O4 nanocrystals.
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dissolution of a larger amount of iron nuclei is advantageous to the growth of ferrite crystallites, thus leading to the formation of ZnFe2 O4 ferrite with larger crystallite size. 3.2. Structural properties of ZnFe2 O4 nanocrystals
Fig. 8. FT-IR spectra for ZnFe2 O4 samples with different crystallite sizes of (a) 9 nm, (b) 14 nm, (c) 19 nm.
The presence of the spinel structure for ZnFe2 O4 samples is further confirmed by FT-IR spectroscopy (Fig. 8). There are two strong characteristic absorption peaks at about 550 and 415 cm−1 , which can be assigned to the stretching vibrations of the Zn–O bonds in tetrahedral positions and the Fe–O bonds in octahedral positions, respectively [31]. In addition, it is noteworthy that with the increase of crystallite size, a slight shift for the stretching vibration of the Zn–O bonds from 553 to 542 cm−1 can be observed. It is well documented that the decrease in crystallite size of ZnFe2 O4 nanoparticles favors the redistribution of Zn2+ cations to octahedral sites in spinel structure [32], in spite of the high affinity of Zn2+ ions in tetrahedral sites. In addition, it is well known that the tetrahedrally coordinated Zn–O distance is shorter than octahedrally coordinated Zn–O distance in cubic spinel structure. Therefore, with the decreasing crystallite size of ZnFe2 O4 nanoparticles, the number of octahedrally coordinated Zn–O bonds increase gradually, thus leading to the observed shift of Zn–O vibration in IR spectra. On the other hand, in view of crystalline electric field effect and size effect, the crystals with smaller crystallite size have more intensive distortion of lattices and higher strength of surface chemical bonds [33], also leading to the change in IR spectra to some extent.
Fig. 9. XPS spectra of 9 nm ZnFe2 O4 sample: (a) survey (b) Zn 2p and (c) Fe 2p.
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where Eg represents the optical band gap, Ep is the photon energy, K is a constant, and n depends on the nature of the transition. In fact, n assumes a value of 1/2, 3/2, 2, and 3 for allowed direct, forbidden direct, allowed indirect, forbidden indirect transitions, respectively. By analysis of absorption coefficients for ZnFe2 O4 , the optical energy gaps can be estimated using a classical Tauc approach [38]. As for three ZnFe2 O4 nanocrystal samples, the best fit of (˛Ep )2 vs Ep was obtained for n = 1/2 (Fig. 11), suggesting the allowed direct transitions across the energy band gap of ZnFe2 O4 . The extrapolated value (the straight lines to the x axis) of Ep at ˛ = 0 gives absorption edge energies corresponding to energy band gap (Eg ) of 2.10 for 9 nm ZnFe2 O4 sample, 2.00 for 14 nm sample and 1.92 eV for 19 nm sample, respectively. The noteworthy feature is a direct relationship between the grain size of the nanoparticles and optical band gap. The shift in Eg of the ZnFe2 O4 nanoparticles with the increasing crystallite sizes is the result of quantum confinement effects arising from the small size regime [38,39]. 3.3. Photo-degradation properties of ZnFe2 O4 nanocrystals Fig. 10. UV–vis diffuse reflectance spectra of ZnFe2 O4 with different crystallite sizes of (a) 9 nm, (b) 14 nm and (c) 19 nm.
The surface/near-surface chemical states of the obtained 9 nm ZnFe2 O4 sample are analyzed by XPS within a range of binding energies of 0–1200 eV (Fig. 9a). Core levels of Zn 2p and Fe 2p can be identified, and no contaminant species are detectable within the sensitivity of the technique. Only a very low peak from absorbed carbon is present on the spectra. The fine spectra of the Zn 2p and Fe 2p peaks are displayed in Figs. 9b and c, respectively. The binding energy of the Zn 2p3/2 peak is 1021.1 eV, indicative of the presence of Zn2+ species. The Fe 2p3/2 and Fe 2p1/2 spectrum obtained from the present study generally shows two distinguishable main peaks at BE of around 711.0 and 724.9 eV, respectively, accompanied by two corresponding satellite peaks for Fe 2p3/2 and Fe 2p1/2 visible at BE of around 718.9 and 733.0 eV, only indicative of the presence of Fe3+ cations [34,35]. Generally, the chemical states of the species characterized by means of an ordinary XPS route only should be considered representative of the surface or external layers composition of the synthesized ferrite samples. However, by integrating the core-level intensities, the obtained Zn/Fe atomic ratio is 0.52, very close to that obtained by ICP and EDS analysis. This further reflects the homogeneous nature of as-synthesized ZnFe2 O4 nanocrystals. To give important information concerning the effect of crystallite size on the optical properties of ZnFe2 O4 semiconductor materials, the UV–vis diffuse reflectance measurement was carried out (Fig. 10). The optical absorbance can be approximately calculated from the optical reflectance data by the Kubelka–Munk function, ˛ = (1 − R)2 /2R, where ˛ is the absorption coefficient, R is the diffuse reflectance [36]. As shown in Fig. 10, the as-synthesized ZnFe2 O4 nanocrystals are highly photoresponsive in both the UV and visible light ranges (>400 nm) as expected. Especially, in the wavelength range of 300–500 nm, three samples present the strong light absorbance (in term of Kubelka–Munk equivalent absorbance units), indicative of wider absorption band than TiO2 . It is well known that when semiconductors are irradiated by photons with energy similar to the band gap, they can yield an electron (e−) in the conduction band and a hole (h+) in the valence band. For a large number of semiconductors, the dependence of the absorption coefficient ˛, for the high-frequency region, upon the photon energy Ep , for optically induced transitions, is usually given by the following expression [37]: ˛Ep = K(Ep − Eg )n
(4)
As discussed above, as-synthesized ZnFe2 O4 nanocrystal is a semiconductor with the optical adsorption threshold in the range of 590–645 nm. Photocatalytic process is based on the generation of electron/hole pairs by means of band-gap radiation, which can give rise to redox reactions with species adsorbed on the surface of the catalysts. In addition, the BET analysis shows that the specific surface areas of ZnFe2 O4 nanocrystals are 54.2, 63.5, 100.5 m2 /g, respectively, with the decrease of crystallite size. Therefore it is likely that due to their suitable band gap from the UV-light response discussed above and high specific surface area, the as-synthesized ZnFe2 O4 nanocrystals produce large amounts of electron/hole pairs under UV irradiation with proper wavelength, and thus may show good photocatalytic performance. ZnFe2 O4 sample with 5 m2 g−1 of specific surface area prepared by the conventional solid-state method at 1000 ◦ C for 12 h is used here as a reference sample, making direct comparison of the catalytic activity of the two kinds of zinc ferrite materials possible. Temporal evolution of the spectral changes taking place during the photo-degradation of AO-II azodye molecule mediated by 9 nm ZnFe2 O4 nanocrystal is displayed in Fig. 12. As expected, a gradual decrease in the intensity of strong absorption band at 484 nm for AO-II solutions due to the n–* transitions of azo bond
Fig. 11. Tauc plot for ZnFe2 O4 with different crystallite sizes of (a) 9 nm, (b) 14 nm and (c) 19 nm.
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Fig. 12. Absorption changes of AO-II solution during the photo-degradation process over 9 nm ZnFe2 O4 sample under UV-light irradiation.
is observed during the course of the photoassisted degradation, indicating that the azo groups has been destroyed [40]. Meanwhile, the other absorption at 309 and 230 nm attributed to the –* transition related to aromatic rings decreases significantly along with irradiation time and no new bands appear, implying that full oxidative decomposition had occurred and that other intermediates are no longer formed. The above result indicates that the ZnFe2 O4 nanocrystals indeed exhibit good activity for the photodegradation of AO-II under UV-light irradiation. The dependence of AO-II photo-degradation on the crystallite size of ZnFe2 O4 nanocrystals under UV-light irradiation has been investigated at the same operating conditions. As shown in Fig. 13, three ZnFe2 O4 nanocrystals with different crystallite sizes exhibit good photo-degradation activity for AO-II, and there are no obvious degradation of dye molecules was observed in the darkness. It is found that the photocatalytic activity is gradually enhanced with decreasing crystallite size. Especially, the degradation percentage of AO-II after 6 h on 9 nm ZnFe2 O4 sample was as high as 97%. The dependence of AO-II photo-degradation on the crystallite size of ZnFe2 O4 nanocrystals can be figured out more clearly from Fig. 14, illustrating that the degradation percentage of
Fig. 14. Influence of the crystallite size of ZnFe2 O4 sample on the degradation percentage of AO-II and kinetic rate constant after 6 h under UV-light irradiation.
AO-II and kinetic rate constant k (first-order) basing on the Langmuir–Hinshelwood model increase obviously with the crystallite size of ZnFe2 O4 nanocrystals. Firstly, it is mainly because the generated conduction band of nanosized ZnFe2 O4 semiconductor with the larger optical band gap is more negative and the valence band is more positive, which makes the oxidation and reduction abilities of photogenerated holes increasing [8,41]. Secondly, the smaller size can lead to a shorter time for the electron diffusion from bulk phase to interface, hence decreases the probability of holes coupling with electrons [42,43]. Finally, the higher surface areas also facilitates to the absorption of dye molecules on the surfaces of ZnFe2 O4 , which makes the free radical more chance to touch with dye molecules thus giving a faster reaction speed. On the other hand, it should be noted that all ZnFe2 O4 samples prepared by modified reduction–oxidation route show higher conversion of AOII dye during the course of photo-degradation, as compared to bulk ZnFe2 O4 sample prepared by the conventional solid-state method, due to the large particle size and low surface area of bulk ZnFe2 O4 sample. In addition, as discussed above, as-synthesized ZnFe2 O4 nanocrystals can be excited under visible light, which expands their utilization scope. Further work on the visible-light derived photocatalytic properties of nanocrystalline ZnFe2 O4 materials is currently underway in our laboratory. 4. Conclusions
Fig. 13. Photo-degradation of AO-II monitored as the normalized concentration change vs irradiation time using series samples under UV-light irradiation: (a) 9 nm ZnFe2 O4 , (b) 14 nm ZnFe2 O4 , (c) 19 nm ZnFe2 O4 , (d) bulk ZnFe2 O4 and (e) dark reaction with 9 nm ZnFe2 O4 .
Zinc ferrite nanocrystals with the average crystallite sizes of 9, 14 and 19 nm were successfully prepared by a modified reduction–oxidation method, which involves a very rapid mixing of ferric cations with reducing agent and nucleation of metallic iron by reduction within a very short time in a colloid mill reactor, followed by slow oxidation of iron and structural transformation under hydrothermal conditions. It is believed that the extreme forces, to which the nucleation of metallic iron obtained with a very short time was subjected in the colloid mill, prevented aggregation of the newly formed iron nuclei. Consequently, when the resulting iron nuclei were slowly oxidized in a Zn2+ -containing aqueous solution under separate hydrothermal treatment, wellcrystallized ZnFe2 O4 nanocrystals with narrow size distribution were obtained. Meanwhile, the photo-degradation investigation reveals that as-synthesized ZnFe2 O4 nanocrystals displayed good abilities to photodecompose acid orange II under UV irradiation,
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and that the reduced crystallite sizes of ZnFe2 O4 nanocrystals can lead to an enhanced photocatalytic activity, proceeding from the quantum confinement effect and high specific surface area structure. The findings in this work are believed to provide further insights to understanding of relation between the structure of ZnFe2 O4 nanocrystals and their photocatalytic properties, which is significantly crucial to generate the desired photocatalyst. It can be expected that as-synthesized zinc ferrite nanocrystals with excellent structural properties, such as high thermal stability and strong resistance to acid and alkali, may have potential applications in the field of industrial photo-degradation of organic azodye pollutants. Acknowledgements We gratefully thank the financial support from the National Natural Science Foundation of China, 973 Program (2009CB939802), 111 Project (No. B07004) and Changjiang Scholars and Innovative Research Team in Universities (IRT 0406). References [1] P. Boldrin, A.K. Hebb, A.A. Chaudhry, L. Otley, B. Thiebaut, P. Bishop, J. Darr, Ind. Eng. Chem. Res. 46 (2007) 4830–4838. [2] H.H. Hamdeh, Z. Xia, R. Foehrweiser, B.J. McCormick, R.J. Willey, G. Busca, J. Appl. Phys. 76 (1994) 1135–1140. [3] W.Z. Xiong, G.M. Kale, Anal. Chem. 79 (2007) 3561–3567. [4] H. Deng, X.L. Li, Q. Peng, X. Wang, J.P. Chen, Y.D. Li, Angew. Chem. Int. Ed. 44 (2005) 2782–2785. [5] C.W. Yao, Q.S. Zeng, G.F. Goya, T. Torres, J.F. Liu, H.P. Wu, M.Y. Ge, Y.W. Zeng, Y.W. Wang, J.Z. Jiang, J. Phys. Chem. C 111 (2007) 12274–12278. [6] J.J. Liu, G.X. Lu, H.L. He, H. Tan, T. Xu, K. Xu, Mater. Res. Bull. 31 (1996) 1049–1056. [7] J.A. Toledo-Antonio, N. Nava, M. Martínez, X. Bokhimi, Appl. Catal. A 234 (2002) 137–144. [8] J. Qiu, C. Wang, M. Gu, Mater. Sci. Eng. B 112 (2004) 1–4. [9] Z.H. Yuan, L.D. Zhang, J. Mater. Chem. 11 (2001) 1265–1268. [10] M. Kobayashi, H. Shirai, M. Nunokawa, Ind. Eng. Chem. Res. 41 (2002) 2903–2909. [11] M. Tsukada, K. Abe, Y. Yonemochi, A. Ameyama, H. Kamiya, S. Kambara, H. Moritomi, T. Uehara, Powder Technol. 180 (2008) 232–238. [12] M. Sivakumar, T. Takami, H. Ikuta, A. Towata, K. Yasui, T. Tuziuti, T. Kozuka, D. Bhattacharya, Y. Iida, J. Phys. Chem. B 110 (2006) 15234–15243.
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